The present invention relates to a continuous bioprocess for the solubilization of phosphate from ore containing phosphate.
Because soluble phosphate rapidly becomes insoluble during contact with soil solutions and solids and is also immobilized by biochemical processes, phosphate is a non-recoverable, irreplaceable element for plant nutrition and industrial applications. It therefore is necessary to mine phosphate, process it into a soluble form, and routinely reapply it to soil as a fertilizer. The principal markets for phosphate include the fertilizer industry and the production of elemental phosphorous which is used in the chemical and food industries. There are no existing substitutes for phosphate in these designated uses. The uses of phosphate preclude it from being recycled and therefore the supply of phosphate must continually be replenished through the process of mining and purification. It is estimated that approximately 35 million metric tons of phosphate containing ore will be processed each year.
Presently two methods for processing phosphate (PO.sub.4) from phosphate containing apatite ore, or rock, are utilized, viz.: (a) the phosphoric acid wet process and (b) the phosphoric acid oxidation process The energy requirements for these two processes are approximately 9.57 MBtu/ton and 16.04 MBtu/ton of ore respectively. The most energy intensive part of the process is the separation of phosphate from apatite ore. Depending on the method used, this portion of the process demands 54% to 78% of the total processing energy. The wet process furnishes approximately 75% of the phosphoric acid produced in the United States while approximately 25% of the phosphoric acid is produced by the oxidation method. Because of the purity requirements, elemental phosphorous is typically produced by means of the more energy intensive oxidation process. Furthermore, the wet process uses large quantities of sulfuric acid for phosphate fertilizer production at a cost in excess of 3 billion dollars per year and results in the generation of substantial quantities of hazardous waste which are regulated under the Resource Conservation and Recovery Act. Treatment and disposal of these hazardous wastes involve a significant expense to the phosphate manufacturing industry and these costs are ultimately borne by the consumers. A biological processing approach for extracting phosphate from phosphate containing ore offers a less expensive and less energy intensive means than existing processes for the production of phosphoric acid.
Biological separation technology is presently being exploited with much success in the non-ferrous mining industry for the leaching of low grade copper, gold and uranium sulfide ores. Additionally, the microbial immobilization of solubilized phosphorous is shown by U.S. Pat. No. 3,980,557, Yall et al., and U.S. Pat. No. 4,220,527, Udaka et al., which disclose methods for removing soluble phosphorous from waste water using microorganisms. However, neither the processes used for sulfide ore or those described by Yall and Udaka, addresses the solubilization of phosphate from a material containing insoluble forms of phosphate, as does the present invention.
It has been shown that as a result of chemical precipitation and biological immobilization, without periodic replacement the accessible supply of phosphate in soil would rapidly be depleted. This would be the case if it were not for the occurrence of biological and chemical phosphate releasing mechanisms in soil. Microbial solubilization of soil phosphate was demonstrated and shown to enhance plant growth by Gerretson, F. C., The Influence of Microorganisms on the Phosphate Intake by the Plant, Plant and Soil, vol. 1, pp. 51-81, 1948. To date however, this phenomenon has only been used in situ in soils or batch shake flask experiments and not developed into a continuous process. Because of the known future need for phosphate and the evidence of susceptibility to microbial alteration, phosphate ore is an ideal candidate for a continuous bioseparation process.
It is known that microbes will solubilize phosphate from such sources as dicalcium and tricalcium phosphate, hydroxyapatite, basic slag and rock phosphate. Microorganisms possessing the ability to solubilize phosphate include bacteria, fungi and actinomycetes, and the range of phosphate solubilization ability within such a heterogeneous group is very large. The simpler calcium phosphate compounds appear to be more susceptible to microbial attack than phosphate contained in complex matrices. Studies report that over 50% of the phosphate in dicalcium and tricalcium (TCP) forms can be released by microbes growing in solution while only 1-33% of the phosphate contained in rock phosphate is released. (Louw et al., A Study of Soil Bacteria Dissolving Certain Mineral Phosphate Fertilizers and Related Compounds, Journal of Applied Bacteria, vol. 22, pp. 227-233, 1959; Singh et al., Solubilization of Insoluble Phosphates by Mesophilic Fungi, Rev. Ecol. Biological Science, vol. 19, pp. 17-25, 1982; Kucey, Phosphate-Solubilizing Bacterial and Fungi in Various Cultivated and Virgin Alberta Soils, Canadian, Journal of Soil Science, vol. 63, pp. 671-678, 1983; and Singh el al., Solubilization of Rock Phosphated by Phosphate Solubilizers in Broth, Current Science, vol. 53, pp. 1212-1213, 1984). However, Applicant has found in batch shake tests that over 90% of TCP can be solubilized with as much as 85% solubilization of rock phosphate. Applicant has also demonstrated this same enhanced degree of solubilization with an increased concentration of ore under continuous bioprocess conditions.
In addition to using a biological system in the phosphate ore process stream to supplement or replace the present extraction methodologies, such a system could also be used to extract phosphate from waste phosphate ore. By current industrial standards, ore containing less than 26% phosphorous (as P.sub.2 O.sub.5) is considered waste. Typically this waste is stock piled and used as back-fill material at spent mining pits. The microbial process of the present invention would enable the further refinement of this type of material. The microbial process of the present invention can also be used to remove trace or nuisance amounts of phosphate from iron ores and other metal oxides ores, thereby facilitating the processing of the metallic ores. Furthermore, an in situ field-leaching process for recovery of phosphorous can be accomplished using the bioprocess of the present invention.
It is an object of this invention to provide a continuous biotechnical method for the industrial phosphate recovery process for the economical removal of phosphate from rock phosphate.
It is another object of this invention to provide a biotechnical method for an industrial phosphate recovery process for the removal of phosphate from low grade phosphate ore and waste material containing phosphate.
It is a further object of this invention to provide an in situ bioseparation phosphate recovery process.
It is still a further object of this invention to provide a biotechnical method for the removal of nuisance amounts of PO.sub.4 from iron ores and other metal oxide ores.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.